In railroad rolling stock it is known to employ trucks to support railroad car bodies during motion along railroad tracks. Commonly, a rail road car truck may have a pair of side frames, or side frame assemblies, that seat upon wheelsets, and a truck bolster that extends crosswise between, and is resiliently mounted to, the side frames. The bolster may typically have a centerplate bowl located at mid-span. The car body may include a centerplate that seats in the centerplate bowl in a relationship that permits a vertical load from the car body to be passed into the truck bolster while also permitting rotational pivoting of the bolster relative to the car body such that the truck can turn and the rail road car can negotiate curves in the track.
As a first approximation, at the simplest level of analysis, the truck bolster may be considered to be a simply supported beam. The car body and lading may be idealized as a vertically downward point load applied at the mid-span center of the beam. This point load is reacted by a pair of reactions, which may for initial approximation also be idealized as point loads, that act vertically upwardly at the beam ends, those reactions being provided by the main spring groups. The main spring groups have upper seats on the undersides of the ends of the bolster, and lower seats on the tension member of the side frames. Truck bolsters may tend to have the general form of a beam having a top flange, a bottom flange, and shear webs extending between the top and bottom flanges. The bending moment in the truck bolster may tend to be greatest at mid span. Consequently, the beam may tend to be deepest in section at the mid span location. While welded or riveted truck bolsters are known, truck bolsters tend commonly to be castings, most typically steel castings.
Truck bolsters may have side bearings mounted on their upper flanges some distance outboard from the centerplate. The side bearings receive vertical loads that are transmitted, typically, between a body bolster of the railroad car body, and the truck bolster. This may tend to occur most particularly when the car body is in a condition where it may lean to one side relative to the truck bolster. The side bearing may include a roller or a slider that permits this transfer of force to occur while also permitting a turning, or pivoting motion of the truck bolster relative to the body bolster. When the railroad car body is in a rocking or leaning condition, the vertical force transmitted into the side bearing, and hence into the bolster arm beneath the side bearing, can be quite substantial.